1. Field of the Invention
This invention relates generally to a heterojunction bipolar transistor with phosphorus doping and more particularly to a heterojunction bipolar transistor with phosphorus doping to prevent diffusion of beryllium atoms from the collector to the emitter.
2. Discussion of the Prior Art
Some of the reliability problems of HBT""s have been traced to the gallium interstitial atoms formed in the sub-collector region and beryllium atoms which migrate to the emitter during the formation of the device.
In one form of this transistor, the layers are grown in series on top of each other as shown in FIG. 3. The first layer 30 is an N+ InGaAs sub-collector layer which contains a large concentration of silicon atoms. A collector region 32 is formed on top of the sub-collector as an Nxe2x88x92 GaAs layer with a lower concentration of silicon. A P+ base layer 34 is formed thereon of GaAs with beryllium doping. An emitter layer 36 is then formed on top of the base layer.
The crystal structure 10 of a sub-collector of a prior art device is shown in FIG. 1. As shown in FIG. 1, gallium atoms may reside in the crystal structure at interstitial locations. Since these atoms are not tightly bonded to the crystal structure, they often diffuse through the structure, especially during the growth of the other layers. Since the growth is relatively slow, on the order of one layer of atoms per second, the interstitial gallium can diffuse as the device is grown. Problems arise when the interstitial gallium reaches the base layer. In that layer, the interstitial gallium atoms from the sub-collector tends to displace the beryllium atoms from their positions in the base so that they are free to diffuse to the emitter. The result is poor reliability of the device.
Previously this problem has been approached by varying the growth conditions such as temperature and overpressure of the group-V material. However, these types of procedures have not been completely successful.
Accordingly, one object of the invention is to provide a novel HBT device having phosphorus doping in the sub-collector.
A further object of this invention is to improve the reliability of HBT devices by preventing the diffusion of beryllium atoms from the base to the emitter.
Briefly, these and other objects of this invention are achieved by providing phosphorus in the form of doping of the sub-collector layer or by providing a collector layer having phosphorus such as InP.